Plasma sputter etching/deposition systems have long been used in the electronics industry for fabricating various sophisticated items such as LSI and VLSI circuits, memories, magnetic read/record heads, etc. The systems may characteristically be used for the deposition of material onto a target (sputter deposition) or for the selective removal (etching) of material from such a target. The material removal processes remove material by ion or electron bombardment or by reactive ion etching. Typical plasma systems are RF sputtering, magnetron sputtering, diode (DC) sputtering, ion beam sputtering, ion plating, etc. As with any industrial process, any modifications which make the process more efficient either in terms of time or expense greatly improves the value of the process.
The following description of the present invention is directed primarily toward high plasma density magnetron sputter etching/deposition systems, however it is to be understood that the concepts disclosed have broader applicability.
A magnetron sputtering system is basically a diode plasma device with a strong magnetic enhancement at the cathode. This magnetic enhancement serves to form an electron trap, such that electrons undergo ExB drifting paths which close upon themselves. The strong magnetic fields present also increase the electron ionization probability and the plasma density, leading to high ion bombardment rates of the cathode and high sputtering rates. Two basic types of magnetrons have been developed which are: the cylindrical post magnetron, and the planar magnetron.
A magnetron sputtering device may be characterized by two equations. The first, by J. A. Thornton, J. Vac. Sci & Technology, Vol. 15 (1978) pp 171, relates the lowest operating voltage, V, to the average energy required for the production of an electron-ion pair, E: EQU V=E/.gamma.e.sub.1 e.sub.2 ( 1)
where .gamma. is the secondary electron coefficient for ion bombardment (of the cathode), e.sub.1 is the probability that an ion will hit the cathode, and e.sub.2 is the probability that a secondary electron will totally utilize its energy in ionization. The second relevant equation is the empirical relation EQU i=k V.sup.n ( 2)
where V is the operating voltage, i is the magnetron current and n is an exponent in the range of 3-10. Higher values of n indicate more efficient magnetron operation, a value of 5-7 being an average value.
The first equation predicts minimum or turn-on magnetron voltages of over 300-350 volts. Voltages in this range are indeed found experimentally, although the magnetron currents at those energies are quite small and the sputtering and deposition rates low. A typical magnetron sputtering system operates in the 400-600 volt range at currents of a few amps, and up to 20 amps in very large systems (Op Cit., Thornton). The operating pressures of a magnetron device are in the 3-10 millitorr range. At constant power, an increase in the pressure will result in an increase in the magnetron current and a decrease in the operating voltage. At high pressures, though, scattering of the sputtered material becomes even more significant and the actual deposition rate will decrease.
There are several drawbacks with magnetron devices. The first is the operating pressure, which is by necessity in the 3-10 millitorr range. At these pressures, the mean-free path of a sputtered atom is only a centimeter or less. This short length means that sputtered material is often scattered prior to deposition on a substrate. Typically only 35% of the total material removed from the target is deposited on the substrate area, the remaining 65% coat the various parts of the system, as well as being redeposited on the target as shown in (W. H. Class, "Thin Solid Films," Vol. 107, (1983), p 379). This scattering also limits the effective target-to-substrate distance to a few centimeters, which can result in significant substrate electron and ion fluxes. The scattering also results in a loss of directionality of the depositing flux, which makes such processes as "lift-off" more difficult. The second severe operating problem with magnetrons is the high energies necessary for operation. Typical energies of 400-600 eV are needed for useful sputtering rates. These high energies can cause significant target damage, or substrate damage in the case of samples being the sputtering target. To increase the deposition or sputtering rates, it is necessary to increase the magnetron voltage. As the electron energy increases, the ionization probability near the target surface decreases, and the discharge becomes more inefficient. The high energies can induce or inhibit various chemical reactions at the target surface, which may not necessarily be desired. The magnetron sputtering system is restricted by equation (1) to only operating at high voltage, and only with the current/voltage characteristic described by equation (2).
A hollow cathode is a plasma device which is capable of emitting a high electron current. The actual operating procedure is well known and has been described in detail in the (H. R. Kaufman, R. S. Robinson and D. C. Trock, "J. Spacecrafts and Rockets," Vol. 20, (1983), p 77), and will not be repeated here. By biasing the hollow cathode sufficiently negative of some anode, a plasma can be produced due to electron ionization of the background (working) gas. This plasma is characterized by a discharge current, which is also equal to the emission current of the hollow cathode. With even a small hollow cathode of diameter 1/8 inch, discharge currents of up to 15 amps are possible at pressures in the 0.2-0.6 millitorr range in Argon.
The hollow cathode effect per se has been described in great detail in the following three references, as well as quite a few others, and will not be described in detail here:
1. H. R. Kaufman, R. S. Robinson and D. C. Trock, J. Spacecrafts and Rockets, Vol. 20 (1983) p. 77.
2. H. R. Kaufman, in "Advances in Electronics and Electron Physics," Vol. 36, Academic Press, NY, (1974), p. 265.
3. J. L. Delcroix and A. R. Trindade, in "Advances in Electronics and Electron Physics," Vol. 35, Academic Press, NY, (1974), p. 87.
In the past, hollow cathodes have only been reported which are based on a cylindrical geometry, i.e., based on a tube, which is usually a refractory material such as tantalum. The tube often has a constriction at its tip, which serves to increase the internal pressure of the cathode. Usually, an insert of foil or other material is added near the tip. Gas is incident on the cathode from an external supply, which due to the smallness of the aperture causes pressures of up to a Torr inside the tube. A plasma discharge can be generated by biasing a keeper or anode, positive with respect to the cathode. This plasma will exist in a region which is inside the hollow cathode, which will be at much higher pressure and hence have a much greater plasma density than those values outside of the hollow cathode. The ion bombardment of the foil inside the tip, which is insulated by the outer layers of foil, will cause the inner layers of the foil to become quite hot, often 2000 K. At this high temperature, the foil surface can thermionically emit electrons, which causes a greater generation of plasma. Once this increase in plasma density occurs, the relative potential of the keeper or anode with respect to the hollow cathode can be reduced to voltages in the 30-50 volt range.
Multiple hollow cathodes of a sort have also been developed. The cathodes consist of a number of tubes tightly bound together in an outer tube, sharing a common gas and electrical power supply. The multiple tubes serve to restrict the gas conduction through the tubes, allowing for higher current operation at reduced gas flow. These multiple cathodes, however, do not really depart from the above described mode of operation, and are also restricted to the basic cylindrical geometry.
The ability to radically change the geometry of operation is necessary, however, for a number of specialized applications, such as electron injection into magnetron and other high energy plasmas or other large chambers. In addition, the operation of multiple, separated but coupled hollow cathodes is not possible with the classical design, due to gas flow considerations and coupling problems with power supplies.